1. Field of the Invention
The present invention relates to a method of refining zinc.
2. Description of the Related Art
Zinc smelting processes are roughly classified into dry methods and wet methods. In both methods, a sulfide of zinc, exemplified by sphalerite, is first roasted to separate into SO.sub.2 gas and ZnO. The SO.sub.2 gas evolved is recovered in the form of sulfuric acid, and the roasted ore containing ZnO is used as the starting material for zinc production both in the dry methods and in wet methods.
In dry methods, generally employed are New Jersey process employing a vertical retort, St. Joseph process employing vertical electrothermic distillation, and ISP process employing a blast furnace. In any of the processes, carbon such as coke is added to the ZnO-containing roasted ore, and it is heated to reduce the zinc oxide to metallic zinc, which is separated by volatilization and collected at a condenser. A higher purity of metallic zinc is produced by introducing the resulting distilled zinc into a rectification column to separate lead contained in the metallic zinc in a minute amount by utilizing the difference of the vapor pressure between zinc and lead.
In wet methods, the roasted ore containing ZnO is dissolved in sulfuric acid solution. Impurities such as Fe and Mn contained in the solution are separated by a pH separation process by adding alkali or by a jarosite process. Cd is separated from the ZnSO.sub.4 -containing solution by adding metallic zinc powder and causing substitution reaction based on the difference of ionization tendencies of the metal ions. The ZnSO.sub.4 solution, having been subjected to various additional purification, is introduced finally to an electrolysis step.
In the electrolysis step, a large quantity of electrolytic solution is withdrawn for keeping water balance and H.sub.2 SO.sub.4 balance, because, in the electrolysis step, H.sub.2 SO.sub.4 concentration increases due to deposition of ZnO, the Mn ion still remaining after the purification of the solution is required to be removed, and the Na ion concentration have to be kept low. The drawn-out electrolytic solution is recycled for leaching out ZnO from the aforementioned roasted ore to remove impurities.
Such smelting of zinc according to wet methods is disadvantageous in many points. The acid concentration in the raw material solution has to be kept low owing to the necessity of using a purified electrolytic solution for dissolution of the raw material, and special care should be taken for roasting sphalerite ore so as not to cause the decrease of leaching rate. The raw materials are restricted and oxide ores or silicate ores containing higher content of impurity compared with sphalerite cannot be used by the reason that use of NaOH, Na.sub.2 CO.sub.3 and the like for pH separation on purification of the recycling ZnSO.sub.4 solution induces the increase of Na ion concentration in the solution, causing adverse effects on electrolysis and use of Ca-containing substance such as Ca(OH).sub.2 or CaCO.sub.3 causes formation of gypsum which increases the loss of Zn. ZnO.Fe.sub.2 O.sub.3 or the like, if it is formed on roasting sphalerite, requires the use of dilute acid for dissolution. The purification of the recycled liquid will cost much in the case where raw materials containing Fe in higher content than Zn is used. In consideration of industrial wastes as a zinc source other than the natural resources, the zinc content in iron scrap is becoming higher with the increased consumption of zinc-galvanized sheet iron, which causes the increase of zinc content in the wastes, such as remarkable rise of the zinc content in the dusts exhausted from electric furnaces or steel converters using iron scrap, preventing direct recycle of the dust to a blast furnace, resulting in intermixing of Zn in metallic Fe or formation of involatile Zn-ferrite in oxidative atmosphere, and thus complete separation of Zn is extremely difficult even though a method is practiced in which iron raw material is produced by heating in a rotary kiln with a reducing agent such as coke to separate Zn by reduction volatilization and weak reduction of iron. Moreover, the ZnO volatilized and collected has a zinc content as low as 30 to 40% in many cases, so that the ZnO cannot be sold to zinc refineries as a zinc material but is transferred to them with paying expense without additional value to the products.
Further, in case where Zn-containing material is dissolved in a strong acid, though the restriction on the raw materials is lessened and the amount of insoluble matter is decreased, economical production of zinc is not feasible because the neutralization of the acid employed is costly.